linux-kernel - Re: [PATCH v13 01/17] preempt: Track NMI nesting to separate per-CPU counter

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Message-ID: <e2346583-0c70-44a1-b0a1-ed41b1175835@nvidia.com>
Date: Tue, 14 Oct 2025 18:05:56 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Lyude Paul <lyude@...hat.com>, rust-for-linux@...r.kernel.org,
 Thomas Gleixner <tglx@...utronix.de>, Boqun Feng <boqun.feng@...il.com>,
 linux-kernel@...r.kernel.org, Daniel Almeida <daniel.almeida@...labora.com>,
 Danilo Krummrich <dakr@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, Uladzislau Rezki <urezki@...il.com>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Gary Guo <gary@...yguo.net>, Bj??rn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Ingo Molnar <mingo@...nel.org>, Ryo Takakura <ryotkkr98@...il.com>,
 K Prateek Nayak <kprateek.nayak@....com>,
 "open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v13 01/17] preempt: Track NMI nesting to separate per-CPU
 counter

On 10/14/2025 3:43 PM, Peter Zijlstra wrote:
> On Tue, Oct 14, 2025 at 01:55:47PM -0400, Joel Fernandes wrote:
>>
>>
>> On 10/14/2025 6:48 AM, Peter Zijlstra wrote:
>>> On Mon, Oct 13, 2025 at 11:48:03AM -0400, Lyude Paul wrote:
>>>
>>>>  #define __nmi_enter()						\
>>>>  	do {							\
>>>>  		lockdep_off();					\
>>>>  		arch_nmi_enter();				\
>>>> -		BUG_ON(in_nmi() == NMI_MASK);			\
>>>> -		__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
>>>> +		BUG_ON(__this_cpu_read(nmi_nesting) == UINT_MAX);	\
>>>> +		__this_cpu_inc(nmi_nesting);			\
>>>
>>> An NMI that nests from here..
>>>
>>>> +		__preempt_count_add(HARDIRQ_OFFSET);		\
>>>> +		if (__this_cpu_read(nmi_nesting) == 1)		\
>>>
>>> .. until here, will see nmi_nesting > 1 and not set NMI_OFFSET.
>>
>> This is true, I can cure it by setting NMI_OFFSET unconditionally when
>> nmi_nesting >= 1. Then the outer most NMI will then reset it. I think that will
>> work. Do you see any other issue with doing so?
> 
> unconditionally set NMI_FFSET, regardless of nmi_nesting
> and only clear on exit when nmi_nesting == 0.
> 
> Notably, when you use u64 __preempt_count, you can limit this to 32bit
> only. The NMI nesting can happen in the single instruction window
> between ADD and ADC. But on 64bit you don't have that gap and so don't
> need to fix it.

Awesome, I will give this a try, thanks a lot Peter!!

 - Joel